The present invention relates generally to semiconductor device packaging, and more particularly to a transfer molding process used to package semiconductor dies on substrates.
Semiconductor packages comprise integrated circuits and devices or dies that are attached or bonded to a top side of substrates or wafers. Typically, electrical interconnects such as vias and wire bonds form pathways to electrically interconnect the semiconductor dies to other devices or to a printed circuit board. Mold compound or encapsulant material forms a protective casing or shape of the finished semiconductor package to protect the delicate and fragile integrated circuits and electrical interconnects from moisture, electrical, physical and other environmental forces and stresses.
The process of packaging the integrated circuits and interconnects with the encapasulant or mold compound, such as epoxy resin, typically involves a transfer molding process. In the transfer molding process, the exposed bond wires, die and substrate are enclosed in a mold cavity formed by top and bottom mold forms. The top and bottom mold forms are clamped together on respective sides of the substrate, and the encapsulant or mold compound is injected into the mold cavity through an injection port. In an attempt to release air and prevent trapped air from forming within the encapsulant in the mold cavity during the transfer molding process, cavity bars with air vents are provided around the periphery of the mold cavity in either or both of the top or bottom mold forms. Although air vents are provided in an attempt to prevent mold voids, the inclusion of air vents introduces additional problems in the packaging process.
For example, the air vents are typically built on the mold chase, i.e., on the cavity bar of the mold forms. However, air may still be trapped in the cavity forming a mold void trap or excess encapsulant may escape through the air vents causing air vent flash that can lead to failure of the semiconductor package and lower mold yield. Another problem is uneven clamping force between the top and/or bottom mold forms and the surfaces of the substrate. The encapsulant or mold compound then may escape out of the mold cavity through any gaps formed from the uneven clamping between the mold form and the surface of the substrate causing air vent flash.
One common problem causing an uneven clamping force is a bulging effect that arises at the surface of the substrate in the air vent when the mold forms are clamped to the surface of the substrate. The bulging effect acts to reduce the cross-sectional area of the air vent and reduces the effectiveness of the air vents. The bulging effect arises from the different hardnesses of the materials of the mold forms and the surface of the substrate.
Typically, the surface of the substrate has a solder mask that is much softer than the harder material of the mold forms, which is usually a steel alloy. The clamping force of the mold forms on the surface of the substrate causes the softer solder mask material to bulge in the channel formed by the drain type air vent cavity bar thereby narrowing the cross-section of the air vent cavity bar and causing an uneven clamping force that reduces the air release effectiveness of the air vent cavity bar design. However, if clamping forces are reduced to eliminate the bulging effect, air vent flash incidence increases.
Other factors contributing to uneven clamping forces that lead to air vent flash and increased reject rates include, for example, wear and tear of air vent depth in the mold forms after repeated chemical cleaning solvent exposure and cavity bar or mold chase warpage. The surface of the air vent area of the cavity bar can become worn due to high clamp shearing pressures and chemical solvents used during sheet cleaning of the mold form after each use. As the mold form becomes worn, the air vent depth may become deeper reducing the effectiveness of the air vent and increasing the occurrence of air vent flash. Another factor of wear and tear is the metallurgy or the grain size of the cavity bars or mold form may increase, grow and expand after repeated exposure to high processing temperatures causing the cavity to warp and create uneven clamping forces.
Another factor contributing to uneven clamping forces arises in packaging systems with mold chase designs used for example with plastic ball grid array (PBGA) semiconductor packages that include a floating plate mechanism that may jam during processing. Such floating plate mechanisms are implemented in an attempt to compensate for the batch variations in substrate strip thickness and to ensure proper clamp force on the substrate to prevent solder mask crack and air vent flash. In such designs, any jam of the floating mechanism of the mold chase plate due for example to foreign matter stuck in the mechanism may cause an uneven clamping force that may result in air vent flash and increased reject rates. Thus, there is a need to address or at least alleviate some of the above problems.